On what basis? That computer plus human has to be safer than the sum of the parts, if both parts are working as intended? There's no logical reason to believe

On what basis? That computer plus human has to be safer than the sum of the parts, if both parts are working as intended? There's no logical reason to believe that must be so. The sum of two things can be less safe than either one individually. If Autopilot suddenly decides to veer into a railing, as it did in Montana, then a human operator may not even be capable of responding quickly enough even if they were totally paying attention.